1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine. More specifically, the present invention relates to exhaust gas purification device which includes an NO.sub.x absorbent for removing NO.sub.x in the exhaust gas of an internal combustion engine.
2. Description of the Related Art
An exhaust gas purification device which disposes a NO.sub.x absorbent in an exhaust gas passage of an internal combustion engine is known in the art. A NO.sub.x absorbent absorbs NO.sub.x in the exhaust gas when the exhaust gas from an engine is at a lean air-fuel ratio, and releases the absorbed NO.sub.x and reduces it when the exhaust gas from the engine is at a rich air-fuel ratio. An exhaust gas purification device of this type is, for example, disclosed in international patent publication WO93-25806. In the exhaust gas purification device in the '806 publication, a NO.sub.x absorbent is used for absorbing NO.sub.x in the exhaust gas of a lean-burn engine which can be operated at an lean air-fuel ratio. The device in the '806 publication is provided with means for detecting the amount of the NO.sub.x absorbed in the NO.sub.x absorbent and monitors the amount of the absorbed NO.sub.x during the lean air-fuel ratio operation of the engine. When the amount of the absorbed NO.sub.x reaches a predetermined value, the device lowers the oxygen concentration in the exhaust gas in order to cause the NO.sub.x absorbent to release the absorbed NO.sub.x and reduce the same using reducing substances such as HC, CO in the exhaust gas (in this specification, the operation which causes the NO.sub.x absorbent to release the absorbed NO.sub.x and reduce the released NO.sub.x to N.sub.2 is referred to as "a regenerating operation"). The device in the '806 publication prevents the NO.sub.x absorbent from being saturated with the absorbed NO.sub.x by performing the regenerating operation when the amount of the NO.sub.x absorbed in the NO.sub.x absorbent reaches the predetermined value.
However, in the device of the '806 publication, there is a problem in that the absorbed NO.sub.x may be released from the NO.sub.x absorbent after the engine has stopped and then diffuse into the atmosphere. As explained later, the maximum amount of NO.sub.x which can be absorbed and held by the NO.sub.x absorbent, i.e., a NO.sub.x saturating amount varies depending on the temperature of the NO.sub.x absorbent. Usually, the NO.sub.x absorbent is disposed in the exhaust gas passage at a position where the temperature of the NO.sub.x absorbent falls in a range where the NO.sub.x saturating amount becomes the maximum value (for example, 300.degree. C. to 500.degree. C.), and the above-noted predetermined value of the absorbed NO.sub.x at which the regenerating operation is commenced is set at a relatively large value (for example, 70 to 80% of the maximum NO.sub.x saturating amount in the above temperature range). Therefore, if the engine is stopped immediately before the NO.sub.x amount absorbed in the NO.sub.x absorbent reaches the above-noted predetermined value, a substantially large amount of NO.sub.x is held in the NO.sub.x absorbent after the engine has stopped. However, since the temperature of the NO.sub.x absorbent decreases due to heat radiation from the NO.sub.x absorbent after the engine has stopped, the NO.sub.x saturation amount (i.e., the maximum amount of NO.sub.x which the NO.sub.x absorbent can hold) decreases. Therefore, when the NO.sub.x saturation amount becomes lower than the amount actually held in the NO.sub.x absorbent due to the cooling of the NO.sub.x absorbent, the amount of NO.sub.x in excess of the NO.sub.x saturating amount is released from the NO.sub.x absorbent and diffuses into the atmosphere without being reduced. The amount of the NO.sub.x diffusing into the atmosphere increases in proportion to the maximum NO.sub.x saturation amount of NO.sub.x absorbent, i.e., a NO.sub.x absorbing capacity of the NO.sub.x absorbent. Therefore, if the NO.sub.x absorbent having a large NO.sub.x absorbing capacity is used, the amount of NO.sub.x diffused into the atmosphere after the engine has stopped also become large.
This problem may not occur if the above-noted predetermined value of the absorbed NO.sub.x at which the regenerating operation is commenced (hereinafter, referred to as "a regenerating amount") is set at a relatively low value. By setting the regenerating amount at lower value, the maximum amount of NO.sub.x absorbed and held in the NO.sub.x absorbent during the engine operation becomes low. Therefore, if the regenerating amount is set at a sufficiently low level, the amount of NO.sub.x held in the NO.sub.x absorbent when the engine is stopped never exceeds the NO.sub.x saturating amount even if the NO.sub.x saturation amount decreases after the engine has stopped due to the cooling of the NO.sub.x.
However, if the regenerating amount is set at a lower value, the NO.sub.x absorbing capacity of the NO.sub.x absorbent cannot be fully utilized. For example, if the regenerating amount is set at, for example, 20% of the maximum NO.sub.x saturating amount instead of conventional 70 to 80%, the frequency of the regenerating operation during the engine operation increases by roughly three times. When the frequency of the regenerating operation increases, problems such as a deterioration in the driveability due to changes in the operating air-fuel ratio of the engine and a worsening of a fuel economy of the engine may occur. Therefore, the solution by setting the regenerating amount at a lower value is not practical.